This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2002-245034 filed on Aug. 26, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method for forming an image with tandem-type multi coloring machine. More particularly, it relates to an image forming apparatus and image forming method for correcting sectional and positional shifts of respective color images to be united in one color image.
2. Description of Related Art
A tandem-type color image forming apparatus has a plurality of image carriers and as the result of it, there may occur positional shifts of respective color images. For example, shape and attachment error of optical parts, attachment error of image carriers, a warp of the entirety of the apparatus due to environmental change such as temperature change or the like cause positional shifts of respective colored toner images formed on a transfer belt. Therefore, there has conventionally been practiced a method such that positional shifts of respective color images are detected using registered patterns to correct the positional shifts. That is, registered patterns of predetermined forms are formed on the transfer belt for respective colors and positions of respective color images on the transfer belt are detected by an optical sensor or the like. Various corrections are taken basing on the detection result to reduce positional shifts of respective colored toner images.
Partial magnification variations, other than errors of entire magnification and shifts of start points of image formation, can be components of positional shifts in main scanning direction. Correction of such partial magnification variations is so-called a partial magnification correction. Conventionally, a partial magnification correction has been carried out by modulating video clock of a light source, thereby to change sizes of respective dots. For example, Japanese laid-open patent publication No. 11-198435 discloses a method of partial magnification correction indicated as follows. That is, main scanning directional magnification variations are detected to calculate how much change is needed for frequency of an output image clock within one line of scanning. Frequency of an output image clock is changed by a constant rate of change width so that the frequency is changed by a calculated value within one line of scanning.
However, the above-mentioned conventional method of partial magnification correction in main scanning direction can cope with only constant and uniform positional shifts regardless of any portions. For example, this method cannot cope with a case such that a left edge and a right edge are extended and center portion is shrunk.
The present invention has been made to solve the above described problems of conventional image formation technology. That is, it is an object of the present invention to provide an image forming apparatus and image forming method capable of partial magnification corrections on various partial variations of magnification as well as constant and uniform positional shift regardless of any sections.
To achieve the object, according to a first aspect of the present invention, there is provided an image forming apparatus being tandem type and forming an image by superimposing images formed at image forming sections of respective colors, the image forming apparatus comprising: a correction memory which stores main scanning directional magnification correction data of respective colors with reference to a reference color; a reference magnification memory which stores main scanning directional magnification of the reference color; an image clock generating section which generates an image clock; a readout unit which reads out main scanning directional magnification correction data of respective colors from the correction memory basing on a reference magnification clock which is equivalent to an image clock modulated basing on main scanning directional magnification of the reference color; a modulated clock generating section which generates modulation clocks for respective colors by modulating an image clock basing on main scanning directional magnification correction data of respective colors read out by the readout unit; and an image formation controlling section which generates control signals to the image forming sections in accordance with modulated clocks of respective colors generated by the modulated clock generating section and image data of respective colors, wherein respective image forming sections conduct image formation in accordance with control signals of respective colors from the image formation control section.
According to a second aspect of the present invention, there is provided an image forming apparatus being tandem type and forming an image by superimposing images formed at image forming sections of respective colors, the image forming apparatus comprising: a correction memory which stores main scanning directional magnification correction data of respective colors with reference to an imaginary color; a reference magnification memory which stores main scanning directional magnification; an image clock generating section which generates an image clock; a readout unit which reads out main scanning directional magnification correction data of respective colors from the correction memory basing on a reference magnification clock which is equivalent to an image clock modulated basing on main scanning directional magnification; a modulated clock generating section which generates modulation clocks for respective colors by modulating an image clock basing on main scanning directional magnification correction data of respective colors read out by the readout unit; and an image formation controlling section which generates control signals to the image forming sections in accordance with modulated clocks of respective colors generated by the modulated clock generating section and image data of respective colors, wherein respective image forming sections conduct image formation in accordance with control signals of respective colors from the image formation control section.
According to a third aspect of the present invention, there is provided image forming method for forming an image by superimposing images formed at image forming sections of respective colors, the image forming method comprising steps of: obtaining main scanning directional magnification of a reference color; obtaining main scanning directional magnification correction data of respective colors with reference to the reference color; reading out main scanning directional magnification correction data of respective colors other than the reference color basing on a reference magnification clock which is equivalent to an image clock modulated by main scanning directional magnification of the reference color; generating modulated clocks of respective colors by modulating an image clock basing on main scanning directional magnification correction data of respective colors read out; generating control signals of respective colors in accordance with modulated clocks of respective colors and image data of respective colors; and forming images of respective colors in accordance with the control signals of respective colors.
According to a fourth aspect of the present invention, there is provided an image forming apparatus being tandem type and forming an image by superimposing images formed at image forming sections of respective colors, the image forming apparatus comprising: a memory which stores image data of respective colors for image formation by each main scanning directional line; an image processing section which conducts image processing in accordance with an input-side clock upon receipt of inputs of image data of respective colors and writing image data after through the image processing in the memory in accordance with an input-side clock; an input-side clock supplying section which supplies an input-side clock to the image processing section and the memory; an image clock generator which generates an image clock; a modulated clock generator which generates modulated clocks of respective colors by modulating image clocks in accordance with main scanning directional magnification correction data of respective colors; and an image formation control section which generates control signals to respective image forming sections by reading out image data of respective colors from the memory in accordance with modulated clocks of respective colors generated at the modulated clock generator, wherein respective image forming sections form images of respective colors in accordance with control signals of respective colors from the image formation control section.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.